Latent image ghosting in photoreceptors is a common problem. Such ghosts have generally been identified as resulting either directly from image-wise exposure or from different amounts of positive charge injected in non-toned and toned image areas. Both result in an arrangement of trapped positive charges that release during re-charging on a subsequent cycle, leading to a pattern of differential charge level that manifests itself in subsequent prints as an image-wise pattern of development, or ghost, of a prior image cycle. Massive erase light is conventionally used to flood the image-wise exposed photoreceptor to remove trapped charges. Such methods generally help, but sometimes prove to be insufficient to suppress ghosting.
The photo-discharge resulting from the erase light, often one or two orders of magnitude greater than the largest exposure utilized during the exposure step of the printing cycle, is always greater when following an image exposure than that observed had the image exposure not preceded the erase, and the magnitude of the increased erase discharge increases with increasing image exposure. One reason for this phenomenon is that not all charge pairs that are photo-generated during image exposure separate, release, and transport out of the photoreceptor layers prior to the erase step. Some portion of these remaining nascent charge pairs is released and transported over time following the initial rapid discharge. These charges are the “delayed release” charges, and manifest themselves as increased dark decay of partially discharged photoreceptors. The remaining portion of these nascent charge pairs continue to be released during the period prior to erase and, depending on their distribution within the generator layer, may contribute to an enhanced local field within the generator, thus augmenting the erase-photogenerated charge yield, resulting in the observed enhanced erase discharge following exposure.
Depending on the physical distribution within the generator layer of these subsequently erase-generated charge pairs, an image-wise population of nascent charges may persist after the first ghost generation cycle charge step, during which they release under the charging field, appearing as increased “depletion” charge, or dark decay, leading to lower charge potential in an image-wise sense. Whether this increased depletion results in positive or negative ghosting depends significantly on the rate of delayed release, the time between exposure and erase, and on the time between erase and charge. If the number density of erase-generated charge in the background is comparable with the image areas, then ghosting may be suppressed. This has traditionally been the rationale behind “flood erase” to suppress image ghosting, in order to minimize the difference in dark decay or depletion between image and background areas going into the first ghost generation. Unfortunately, this traditional approach to suppressing image ghosting has limited success.